In this disclosure, the term “tempered glass sheet article” is used to describe a glass article made from a tempered glass sheet. Similar, the term “glass sheet article” is used to describe a glass article made from a glass sheet. A glass sheet has a top surface, a bottom surface, and an edge connecting the top surface to the bottom surface. Tempering can be used to induce permanent compressive stress or compression on the surfaces of the glass sheet, thereby forming a tempered glass sheet. The depth of the surface layer under compression (also referred to as “depth of layer”) at the surfaces is dependent on the tempering process and the material of the glass sheet. When force is applied to the tempered glass sheet, any tension induced near the glass surface will be reduced by the pre-existing compressive stress at the glass surface. Thus, greater force will be needed to break the tempered glass sheet article, i.e., compared to a glass sheet of the same shape and material that is initially free of such compressive stress. There are two ways of tempering glass: (i) thermal tempering; and (ii) ion-exchange, or chemical, tempering. Of these two ways of tempering glass, greater surface compression can be obtained using the ion-exchange method.
For an ion-exchanged glass sheet having sharp corners between the edge and surfaces of the glass sheet, the corners exhibit a shallower compressive layer depth, a lower surface compression, and a relatively high local tension or tensile stress, in comparison to those areas of the glass sheet that are far away from the corners. These areas of low compression will have a low breaking strength. The edge area adjacent to the sharp corners is significantly smaller than the surface area adjacent to the sharp corners. As a result, the low surface compression, shallow depth of the compressive layer, and high local tension at the sharp corners will disproportionately affect the edge of the glass sheet. In other words, the edge of the glass sheet will have a much lower breaking strength than the surfaces of the glass sheet article and will thus be the weakest part of the glass sheet.
If the edge of the tempered glass sheet has a low breaking strength, the tempered glass sheet as a whole will be vulnerable to damage because cracks can easily form at the edge (e.g., due to impact loading on or bending of the glass sheet article), and any crack formed at the edge will easily propagate across the glass sheet. Thus, the breaking strength of the edge of the tempered glass sheet essentially defines the breaking strength of the entire tempered glass sheet. One solution is to protect the edge of the tempered glass sheet article. However, this solution is not available in applications where the edge of the tempered glass sheet article must be exposed. For applications where the edge of the tempered glass sheet article will be exposed, and to take full advantage of the glass strengthening offered by tempering, it would be necessary to increase the breaking strength of the edge of the tempered glass sheet article, e.g., to be similar to the breaking strength of the surfaces of the tempered glass sheet article.